#include<iostream>
#include<algorithm>
#include <vector>
#include<cmath>
using namespace std;
typedef long long int lont;
int main() {
	float PX, PY, QX, QY; 
	cin >> PX >> PY >> QX >> QY;
	cout << (abs(PX - PY) + abs(QX - QY)) / 2 << endl;
}